1. Technical Field
The present disclosure relates to cross-point nanoarrays which can be used as non-volatile memory structures or as sensors able to be produced using non-conventional lithographic techniques including so-called “soft-lithography”.
2. Description of the Related Art
As is known, non-volatile memories, i.e., memories which are able to retain the stored data even in the absence of a power supply, are today widespread in many areas of electronics. These devices, at present, are formed on silicon substrates using highly advanced technology which is well-established in particular form the point of view of the reliability and compactness achieved: today the technological challenge is to produce memories using a technology with definition lines having a width of 65 nm and even finer.
The cell structure generally has as a memory element a floating gate which is arranged underneath a control gate electrode; operation of the device is associated specifically to the combined biasing of the two gates which are separated by dielectric and arranged over a channel region in the semiconductor between source and drain. The data to be stored physically consists of the electric charge that is accumulated in the floating gate.
The rapid growth of the memory market is providing an impetus for the development of alternative non-volatile memory structures which are potentially able to overcome the technological limits of scalability of silicon structures at an acceptable cost and improve the performance in terms of read/write speed, data retention time, and reduction of the voltages employed during reading and writing [1].
At present various alternative structures which can be used as a non-volatile memory are being studied, as schematically shown in FIG. 1.
Among the structures studies, the most promising, in terms of integrative possibilities, are organic memories which use polymer materials as the active material and in particular wholly organic memories (i.e., memories which can be produced exclusively with organic materials) and hybrid memories (i.e., memories which can be produced using conventional materials, for example ordinary conductive materials in combination with organic materials).
These memory cells may be made with particularly simple structures (so-called cross-point array structures), as schematically shown in the diagram of FIG. 2, and in some cases arranged in vertical stacks, as schematically shown in FIG. 3, being thus possible to multiply the memory density per unit of footprint area. Moreover, organic memories are the only memories which are in theory capable of allowing scaling down to molecular dimensions and therefore potentially achieving an extraordinary degree of compactness.
In these memory structures, an active organic material is arranged between two electrodes made of a conductive material which can be defined in the form of parallel nanorows in a simple cross-point matrix arrangement.
In the area of organic memories there exist two different types of operation, i.e., based on ferroelectric behavior and resistive switching operation, which are illustrated in FIGS. 2 and 3, respectively.
Of these two different types, the most advantageous in terms of compactness are resistive switching memories since, in the case of ferroelectric memories, an auxiliary transistor is associated to each cell to avoid data loss during reading.
Therefore, cross-point nanoarrays, such as those shown in FIGS. 2 and 3, which are based on modification of the electric resistance through the layer of active organic material that separates the electrodes of the two separate orthogonal orders of electrodes, at the cross-over points, may be useful not only as a non-volatile memory structure, but even as a sensing structure, for example able to detect pressure patterns over a relatively large area, for producing user interfaces, for example in the form of keyboards and for similar uses.
Dual-Electrode Polymer Devices
A two-electrode device may be simply produced by means of a sandwich structure comprising a bottom electrode (consisting of Al, Ag, Cu, Ni, doped polysilicon, etc.), a layer of active polymer material deposited for example by means of spin-coating, and a top electrode (consisting of Al, Ag, Cu, Ni, doped polysilicon, etc.) formed for example by CVD (Chemical Vapor Deposition).
A device of this type may function as a memory element for example by applying an electric field across the active organic material at a cross-over point (cell), able to induce formation of conductive paths in the active polymer material or activate a charge transfer or other detectable modification which can be detected in the organic material. Below few examples of polymer materials, their structures, characteristics and the operating principles of devices using them as active materials are given.
Polystyrene
The behavior of atactic polystyrene film between two metal (Al, Au, etc.) electrodes as a non-volatile memory element was already studied in 1976 by Carchano and colleagues [2]. The I-V characteristics demonstrate a transition from a high-resistance state (108 Ohms) to a low-resistance state (10 Ohms) and the reverse transition after the application of a suitable difference in potential. According to the authors, the switching transition between the two states was due to the formation of conductive strands consisting of carbon atoms (C═C) in the polymer between the electrodes.
The most recent research into conductive polymers relate to polystyrene film containing metallic nanoclusters.
Yang Yang et al. [3][4][5] have studied the operation of devices obtained by depositing an organic film between two Al electrodes, as shown in FIG. 4.
The organic film is formed by depositing by means of spin-coating a solution of Au nanoparticles passivated with 1-dodecanthiol (Au-DT NPs, diameter 1.6-4.4 nm), 8-hydroxyquinoline (8HQ) and polystyrene in 1,2-dichlorobenzene [3]. FIG. 5 show the chemical structures of the materials used.
The bottom electrode and top electrode are thermally deposited from vapor phase, while the active layer is deposited from a solution of 0.4% wt % Au-DT NPs, 0.4 wt % 8HQ and 1.2 wt % polystyrene in 1,2-dichlorobenzene. Upon application of an electric field, the device undergoes a transition between two conductive states and may be written, read and erased repeatedly as shown in FIG. 6.
The presence of two different conductive states suggests a change in the distribution of the electrons of the device owing to the action of the electric field. The 8HQ molecules and Au nanoparticles behave as electron donors and electron acceptors, respectively, such that the electric field activates a charge transfer between Au-DT NP and 8HQ. Prior to the transition, there is no interaction between the two domains, but when an electric field is applied to the device, an electron of the highest occupied molecular orbital (HOMO) of the 8HQ is able to pass through the dodecanthiol passivation layer to the Au nanoparticle, as schematically illustrated in FIG. 7.
The most interesting characteristics of this mechanism are the stability of the two states, the fast switching response, the high ON/OFF ratio (105) and the ample opportunity to vary the materials owing to the simplicity of the manufacturing processes. The effect of various materials on the performance of the devices, substituting 8HQ with another conjugated organic compound, 9,10-demethylanthracene, and the aPS with polymethyl methacrylate PMMA was also studied. The resultant devices had an electrical behavior substantially similar to that described above.
The use of different conductive materials, such Au, Cu and ITO, instead of Al, for the electrodes also did not have a decisive effect on the performance of the device.
A WORM (write-once-read-many-times-memory) memory device, consisting of a polystyrene film containing Au nanoparticles passivated with 2-naphthalenethiol (AU-2NT NPs), the structure of which is shown in FIG. 8, was also produced and inserted between Al electrodes [4].
From an analysis of the I-V characteristic of this device shown in FIG. 3 it can be seen that, owing to the effect of the electric field, the device passes from a low conductivity state, where the current which flows through the device depends on the injection of charge from the electrode to the polymer material and is limited by the barrier present at the metal-polymer interface, to a higher conductivity state, where the current is associated with the formation of an excess charge in the organic layer situated between the two electrodes (space-charge-limited current region). The increase in the current between the two states, measured at about 2V, is greater than three orders of magnitude. Upon reversing the polarity of the electric field there was no transition to the “0” state, but instead a significant increase in the absolute value of the current, thereby confirming the existence of a space-charge-limited current regime.
This transition between the two conduction states was reasonably attributed to the activation of a charge transfer by the electric field between the Au nanoparticles and the 2-NT film which passivates them as schematically shown in FIG. 10. In view of the stability of the device in the higher conductivity state, it may be used as a WORM memory.
Polymethyl Methacrylate
The behavior of polymethyl methacrylate, polyethyl methacrylate and polybutyl methacrylate film between two metal electrodes (Al, Au, etc.) for use in non-volatile memory devices was already studied in 1974 [6]. FIG. 11 shows the result obtained on a film with a thickness of about 0.5 μm deposited between two metal electrodes from a solution containing 5% PMMA from butanone or benzene. As in the case of atactic polystyrene, the authors suggest that switching is induced by the formation in the polymer of conductive strands of carbon atoms (C═C) between the electrodes.
The most recent research into the derivatives of methyl methacrylate relate to the use of polymethyl methacrylate film as a material for the matrix containing metal nanoparticles or derivatives of polymethacrylate functionalized with pendant chromophores such as anthracene, poly(methylmethacrylate-co-9-anthracenyl-methylmethacrylate) (10:1), MDCPAC. The use of these polymers allows the excellent mechanical properties of polymethacrylate to be combined with the interesting electronic characteristics of anthracene.
The device is formed by means of vapor phase deposition of the bottom Au electrode on a glass substrate, followed by deposition of the active MDCPAC layer by means of spin-coating from a solution of chloroform (20 mg/mL in ClCH3) and finally vapor phase deposition of the second Al electrode.
The I-V characteristic of such a device is shown in FIG. 12a and FIG. 12b The first graph (FIG. 12a) shows the characteristic of a circuit consisting of the memory device in series with a 107Ω resistance, from where it can be seen that when V=Vcrit the device switches between two states (OFF and ON).
When V<Vcrit the current in the circuit is essentially controlled by the device which has a resistance greater than 107Ω, corresponding to an OFF state. When V>Vcrit, the current in the circuit is controlled essentially by the resistor in series with the device, which is therefore in a lower resistivity state (ON). If the voltage applied is reduced the device remains in the ON state until V=Vhold, which is the erase voltage.